path: root/include/clang/Sema/Ownership.h

//===--- Ownership.h - Parser ownership helpers -----------------*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
//  This file contains classes for managing ownership of Stmt and Expr nodes.
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_CLANG_SEMA_OWNERSHIP_H
#define LLVM_CLANG_SEMA_OWNERSHIP_H

#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/PointerIntPair.h"

//===----------------------------------------------------------------------===//
// OpaquePtr
//===----------------------------------------------------------------------===//

namespace clang {
  class ActionBase;
  class Attr;
  class CXXBaseOrMemberInitializer;
  class CXXBaseSpecifier;
  class Decl;
  class DeclGroupRef;
  class Expr;
  class NestedNameSpecifier;
  class Stmt;
  class TemplateName;
  class TemplateParameterList;

  /// OpaquePtr - This is a very simple POD type that wraps a pointer that the
  /// Parser doesn't know about but that Sema or another client does.  The UID
  /// template argument is used to make sure that "Decl" pointers are not
  /// compatible with "Type" pointers for example.
  template <class PtrTy>
  class OpaquePtr {
    void *Ptr;
    explicit OpaquePtr(void *Ptr) : Ptr(Ptr) {}

  public:
    OpaquePtr() : Ptr(0) {}

    static OpaquePtr make(PtrTy P) {
      OpaquePtr OP; OP.set(P); return OP;
    }

    template <typename T> T* getAs() const {
      return get();
    }

    template <typename T> T getAsVal() const {
      return get();
    }

    PtrTy get() const {
      return llvm::PointerLikeTypeTraits<PtrTy>::getFromVoidPointer(Ptr);
    }

    void set(PtrTy P) {
      Ptr = llvm::PointerLikeTypeTraits<PtrTy>::getAsVoidPointer(P);
    }

    operator bool() const { return Ptr != 0; }

    void *getAsOpaquePtr() const { return Ptr; }
    static OpaquePtr getFromOpaquePtr(void *P) { return OpaquePtr(P); }
  };
}

namespace llvm {
  template <class T>
  class PointerLikeTypeTraits<clang::OpaquePtr<T> > {
  public:
    static inline void *getAsVoidPointer(clang::OpaquePtr<T> P) {
      // FIXME: Doesn't work? return P.getAs< void >();
      return P.getAsOpaquePtr();
    }
    static inline clang::OpaquePtr<T> getFromVoidPointer(void *P) {
      return clang::OpaquePtr<T>::getFromOpaquePtr(P);
    }
    enum { NumLowBitsAvailable = 0 };
  };
}



// -------------------------- About Move Emulation -------------------------- //
// The smart pointer classes in this file attempt to emulate move semantics
// as they appear in C++0x with rvalue references. Since C++03 doesn't have
// rvalue references, some tricks are needed to get similar results.
// Move semantics in C++0x have the following properties:
// 1) "Moving" means transferring the value of an object to another object,
//    similar to copying, but without caring what happens to the old object.
//    In particular, this means that the new object can steal the old object's
//    resources instead of creating a copy.
// 2) Since moving can modify the source object, it must either be explicitly
//    requested by the user, or the modifications must be unnoticeable.
// 3) As such, C++0x moving is only allowed in three contexts:
//    * By explicitly using std::move() to request it.
//    * From a temporary object, since that object cannot be accessed
//      afterwards anyway, thus making the state unobservable.
//    * On function return, since the object is not observable afterwards.
//
// To sum up: moving from a named object should only be possible with an
// explicit std::move(), or on function return. Moving from a temporary should
// be implicitly done. Moving from a const object is forbidden.
//
// The emulation is not perfect, and has the following shortcomings:
// * move() is not in namespace std.
// * move() is required on function return.
// * There are difficulties with implicit conversions.
// * Microsoft's compiler must be given the /Za switch to successfully compile.
//
// -------------------------- Implementation -------------------------------- //
// The move emulation relies on the peculiar reference binding semantics of
// C++03: as a rule, a non-const reference may not bind to a temporary object,
// except for the implicit object parameter in a member function call, which
// can refer to a temporary even when not being const.
// The moveable object has five important functions to facilitate moving:
// * A private, unimplemented constructor taking a non-const reference to its
//   own class. This constructor serves a two-fold purpose.
//   - It prevents the creation of a copy constructor that takes a const
//     reference. Temporaries would be able to bind to the argument of such a
//     constructor, and that would be bad.
//   - Named objects will bind to the non-const reference, but since it's
//     private, this will fail to compile. This prevents implicit moving from
//     named objects.
//   There's also a copy assignment operator for the same purpose.
// * An implicit, non-const conversion operator to a special mover type. This
//   type represents the rvalue reference of C++0x. Being a non-const member,
//   its implicit this parameter can bind to temporaries.
// * A constructor that takes an object of this mover type. This constructor
//   performs the actual move operation. There is an equivalent assignment
//   operator.
// There is also a free move() function that takes a non-const reference to
// an object and returns a temporary. Internally, this function uses explicit
// constructor calls to move the value from the referenced object to the return
// value.
//
// There are now three possible scenarios of use.
// * Copying from a const object. Constructor overload resolution will find the
//   non-const copy constructor, and the move constructor. The first is not
//   viable because the const object cannot be bound to the non-const reference.
//   The second fails because the conversion to the mover object is non-const.
//   Moving from a const object fails as intended.
// * Copying from a named object. Constructor overload resolution will select
//   the non-const copy constructor, but fail as intended, because this
//   constructor is private.
// * Copying from a temporary. Constructor overload resolution cannot select
//   the non-const copy constructor, because the temporary cannot be bound to
//   the non-const reference. It thus selects the move constructor. The
//   temporary can be bound to the implicit this parameter of the conversion
//   operator, because of the special binding rule. Construction succeeds.
//   Note that the Microsoft compiler, as an extension, allows binding
//   temporaries against non-const references. The compiler thus selects the
//   non-const copy constructor and fails, because the constructor is private.
//   Passing /Za (disable extensions) disables this behaviour.
// The free move() function is used to move from a named object.
//
// Note that when passing an object of a different type (the classes below
// have OwningResult and OwningPtr, which should be mixable), you get a problem.
// Argument passing and function return use copy initialization rules. The
// effect of this is that, when the source object is not already of the target
// type, the compiler will first seek a way to convert the source object to the
// target type, and only then attempt to copy the resulting object. This means
// that when passing an OwningResult where an OwningPtr is expected, the
// compiler will first seek a conversion from OwningResult to OwningPtr, then
// copy the OwningPtr. The resulting conversion sequence is:
// OwningResult object -> ResultMover -> OwningResult argument to
// OwningPtr(OwningResult) -> OwningPtr -> PtrMover -> final OwningPtr
// This conversion sequence is too complex to be allowed. Thus the special
// move_* functions, which help the compiler out with some explicit
// conversions.

namespace llvm {
  template<>
  class PointerLikeTypeTraits<clang::ActionBase*> {
    typedef clang::ActionBase* PT;
  public:
    static inline void *getAsVoidPointer(PT P) { return P; }
    static inline PT getFromVoidPointer(void *P) {
      return static_cast<PT>(P);
    }
    enum { NumLowBitsAvailable = 2 };
  };
}

namespace clang {
  // Basic
  class DiagnosticBuilder;

  // Determines whether the low bit of the result pointer for the
  // given UID is always zero. If so, ActionResult will use that bit
  // for it's "invalid" flag.
  template<class Ptr>
  struct IsResultPtrLowBitFree {
    static const bool value = false;
  };

  /// ActionBase - A small part split from Action because of the horrible
  /// definition order dependencies between Action and the smart pointers.
  class ActionBase {
  public:
    /// Out-of-line virtual destructor to provide home for this class.
    virtual ~ActionBase();

    // Types - Though these don't actually enforce strong typing, they document
    // what types are required to be identical for the actions.
    typedef OpaquePtr<DeclGroupRef> DeclGroupPtrTy;
    typedef OpaquePtr<TemplateName> TemplateTy;
    typedef Attr AttrTy;
    typedef CXXBaseSpecifier BaseTy;
    typedef CXXBaseOrMemberInitializer MemInitTy;
    typedef Expr ExprTy;
    typedef Stmt StmtTy;
    typedef TemplateParameterList TemplateParamsTy;
    typedef NestedNameSpecifier CXXScopeTy;
    typedef void TypeTy;  // FIXME: Change TypeTy to use OpaquePtr<N>.

    /// ActionResult - This structure is used while parsing/acting on
    /// expressions, stmts, etc.  It encapsulates both the object returned by
    /// the action, plus a sense of whether or not it is valid.
    /// When CompressInvalid is true, the "invalid" flag will be
    /// stored in the low bit of the Val pointer.
    template<class PtrTy,
             bool CompressInvalid = IsResultPtrLowBitFree<PtrTy>::value>
    class ActionResult {
      PtrTy Val;
      bool Invalid;

    public:
      ActionResult(bool Invalid = false) : Val(PtrTy()), Invalid(Invalid) {}
      ActionResult(PtrTy val) : Val(val), Invalid(false) {}
      ActionResult(const DiagnosticBuilder &) : Val(PtrTy()), Invalid(true) {}

      // These two overloads prevent void* -> bool conversions.
      ActionResult(const void *);
      ActionResult(volatile void *);

      PtrTy get() const { return Val; }
      void set(PtrTy V) { Val = V; }
      bool isInvalid() const { return Invalid; }

      const ActionResult &operator=(PtrTy RHS) {
        Val = RHS;
        Invalid = false;
        return *this;
      }
    };

    // This ActionResult partial specialization places the "invalid"
    // flag into the low bit of the pointer.
    template<typename PtrTy>
    class ActionResult<PtrTy, true> {
      // A pointer whose low bit is 1 if this result is invalid, 0
      // otherwise.
      uintptr_t PtrWithInvalid;
      typedef llvm::PointerLikeTypeTraits<PtrTy> PtrTraits;
    public:
      ActionResult(bool Invalid = false)
        : PtrWithInvalid(static_cast<uintptr_t>(Invalid)) { }

      ActionResult(PtrTy V) {
        void *VP = PtrTraits::getAsVoidPointer(V);
        PtrWithInvalid = reinterpret_cast<uintptr_t>(VP);
        assert((PtrWithInvalid & 0x01) == 0 && "Badly aligned pointer");
      }

      // These two overloads prevent void* -> bool conversions.